White LED strings are widely used as backlight of liquid crystal displays (“LCDs”) in computers, televisions, and other electronic devices. Currently LED backlight power supplies typically use a three-stage driver system. As shown in FIG. 1, the driver system comprises a power factor correction (“PFC”) stage 11, an isolated DC-DC (direct current to direct current) voltage converter stage 12, and an LED step-up driver stage 13. The PFC stage rectifies 220V or 110V AC (alternating current) voltage to about 400V or 200V DC (direct current) line voltage. The isolated DC-DC converter stage 12 comprises an isolated voltage converter to convert the DC line voltage to another DC voltage, for example, 120V, and thus provides an input voltage to the LED step-up driver stage 13.
The LED step-up driver stage 13 comprises one or more non-isolated boost converters to receive the output voltage/voltages from the isolated DC-DC voltage converter stage 12. Via the boost converter, a constant current is supplied to an LED string 14. The isolated DC-DC voltage converter stage 12 further transforms the line voltage to a lower DC voltage, for example 18V, which is further converted by system power converters 15 to multi-rail output DC voltages such as 12V and 5V for the system power supply of the controller devices.
The conventional LED driver system in FIG. 1 comprises multiple converters such as isolated converters, non-isolated converters, and system power converters 15. Each of these converters require a controller for control. Accordingly, the conventional LED driver system is complex, has a large size and high power consumption.